The hepatitis A virus (HAV) is a picornavirus that was isolated in cell culture for the first time in 1979. We have succeeded in isolating virus directly form human clinical materials without the intervening serial passage in marmoset monkeys that was necessary for the initial isolation in cell culture. Cell culture-adapted HAV was thrice cloned by terminal dilution. Since the virus is largely cell-associated and generally is not cytopathic in cell culture, cloning was achieved by terminal dilution and radioimmunoassay. The objective of this project is to develop a live attenuated hepatitis A vaccine suitable for administration to high-risk populations in the United States and abroad. Our strategy involves comparison of the response of susceptible primates (chimpanzees and marmosets) to wild type HAV and to tissue culture adapted mutants of HAV. Attenuation of HAV occurs rapidly during serial passage in cell culture and initially the appropriate degree of attenuation must be determined by trial and error. Because primary African green monkey kidney cells that are free of contamination with latent viruses are difficult to obtain, and because of the recent finding of AIDS-like retroviruses in African green monkeys, we have elected to adapt the attenuated HAV to MRC- 5 cells, a semi-continuous cell type that is also licensed for vaccine development. Adaptation and recloning of the virus have been achieved and retesting of the virus in primates is in progress. The MRC-5 adapted virus was completely attenuated for chimpanzees and marmosets when inoculated i.v. at a dose of 106 TCID50. Plans are underway to begin clinical testing in volunteers in collaboration with the Army (WRAIR).